PULSATION WITHOUT MARGINAL SENSE-ORGANS. 25 



(annulus) of sub-umbrella tissue ; and such a ring can readily be set 

 into sustained pulsation. 



It is not necessary, however, that cuts be made through the sub- 

 umbrella tissue ; for mere pressure prevents the transmission of con- 

 traction waves across the pressed region, and we may form circuits by 

 pressing lightly upon the sub-umbrella with a concentric series of 

 metallic rings, as is shown in figure 6. Then upon stimulating the 

 disk in any manner it pulsates rhythmically. 



Disks which have been cut, or pressed, as described above do not 

 pulsate until they have been momentarily stimulated at some definite 



Figs. 8- 19a, Shapes cut from disks without marginal sense-organs. These will pulsate 

 continuously in sea-water. 



point by a touch of some potassium or sodium salt, a mechanical or 

 electrical shock, or by suddenly cutting off the last remaining sense- 

 organ immediately after it has sent out its contraction wave. 



A contraction wave travels outward from the stimulated place 

 through the circuit of sub-umbrella tissue, and when it returns to the 

 point whence it started it is immediately reinforced, and again sent 



